public class OnlyOne {
    public static int only (int[] arr) {
        int c = 0;
        for(int i = 0;i < arr.length;i++) {
            c ^= arr[i];
        }
        return c;
    }
    public static void main(String[] args) {
        int[] arr = {1,1,2,2,3,4,3,5,6,5,6};
        System.out.println(only(arr));
    }
}
